Flexography is today one of the most important processes for printing, particularly for printing for packaging. It uses a flexible photopolymer or rubber printing plate that carries the printing image in relief. The ink delivery system for flexography is achieved via an “anilox” engraved transfer roll.
A photopolymer flexo printing plate is either produced by a process that includes exposing a film on an imagesetter and using the film to expose and process a flexo printing plate, or by the computer-to-plate CTP process.
In the a film-based process, a film negative or positive is imaged in an imagesetter, the film is laid over the photopolymer flexographic plate, and the plate is exposed through the film using ultraviolet (UV) radiation to transfer the imaging information from the film to the plate. The flexo plate is then processed to remove the unexposed areas and generate a three-dimensional relief carrying the image (printing) information.
In a computer-to-plate process, no film is used. The photopolymer flexo plate is made photo-sensitive, typically by laminating a very thin—3 μm or so—black carbon, laser-ablatable layer over a flexo plate. Such a plate is sometimes called a “digital” flexo plate. The carbon layer is sensitive to high power thermal laser light. Therefore, the imaging information is transferred directly to the flexo plate using an imagesetter containing a high power laser source. The flexo plate may now be exposed to UV radiation, the unablated material washed, and the plate further processed in a manner similar to that for a film-based process.
Flexo plates are today expensive compared to other printing media, for example offset printing plates or film. It is therefore desirable to reduce flexo plate waste. Such waste may be severe in applications where only a fraction of overall image is covered by printing information. One application with such sparse images is the printing of corrugated material such as used for making corrugated paper packages, such as boxes. Only roughly 20-30% or even less of a full-format flexo plate for such a printing application might contain printable information, so that about 70-80% of such a full format flexo plate might be used not for transferring printing information, but rather for carrying those parts that do contain imaging information and keeping those parts in accurate relative positions, i.e., in registration.
There thus is an incentive to reduce such waste. One prior-art method to reduce plate waste in flexography is described in U.S. Pat. No. 5,846,691 to Cusdin, et al. (issued Dec. 8, 1998). Prior to imaging, only those elements of the flexo plate that will receive printing information are pre-mounted on the correct position on a suitable carrier sheet. After mounting, the printing information is transferred to flexo plate “patches” on the carrier sheet.
The Cusdin, et al. method has some serious shortcomings. The plate patches are already mounted on a full-format carrier sheet prior to entering the UV exposer, washer, dryer and finisher units. Imagine a corrugated box that might be two meters wide when flattened out to a sheet. To expose and process such a large sheet presents difficulty. Full format imagesetting and processing equipment is very expensive, so using such equipment to print a small part of the overall sheet seems wasteful. Furthermore, carrier sheets are prone to shrinkage/expansion as a result of one or more of the chemical washing, the exposing to intense UV light, and the exposing to heat in a dryer unit. Furthermore, adhesive tape typically is used to adhere the plate patches on the carrier sheet, and such tape can be destroyed during processing. Usually, a time-consuming plate-edge sealing step is therefore added.
Imaging of flexo plates is usually carried out on an external drum imagesetter. When a carrier sheet carrying flexo plate patches, such as produced by the Cusdin, et al. method is thus imaged, serious balancing problems may occur.
Finally, the method of Cusdin is typically practical only for a computer to plate method. Film copying over a patched carrier sheet is almost impossible to execute since the film to plate patch combination cannot be sucked by vacuum, which is required in a typical film-based process.
Thus there is a need for an alternate to patching plate materials on a carrier prior to imaging.
Another known method to reduce plate wastage is called “post-mounting” herein. A full-format film of the design is imaged, and afterwards the film parts that contain information are manually cut out. Only these cut film parts are used to image small flexo plate parts (patches). Producing the patches can use smaller less expensive imaging and processing equipment. The patches are then mounted post-exposure and post-processing in correctly registered positions on a stable carrier sheet, e.g., a Mylar sheet or a flexographic sleeve, using either two-sided tape or another adhesive material. Registering and mounting the patches is aided by special mounting support devices that typically include two or more video cameras and monitors for registering printed register marks. Examples of mounting devices include the Cyrel® Macroflex mounting device from E. I. Du Pont de Nemours and Company, Wilmington, Del. and the Mount-O-Matic® series of plate mounting machines produced by AV Flexologic NV, Alphen aan den Rijn, the Netherlands. Optical devices using duplicate drums and a mirror also are known to help mount the patches.
Such registration devices are expensive. The registration is completely manual because the cutting process is typically inaccurate. It is desired to eventually automate the whole process, including keeping track of relative positioning of the individual patches.
A known variant of the post-mounting method is to cut out the patches “electronically” on a computer aided design station rather than from a large film. The plate parts that contain print information are manually identified by an operator on a computer screen. Identification of the patches takes place when the print job data is still in editable file format (e.g. PostScript, PDF or similar formats). The individual plate patches—still stored in editable data format—are individually sent through a raster image processor (RIP) to convert them into a format that can be output to a film imagesetter or a computer-to-plate device. The parts are then imaged and processed individually prior to mounting.
Because each plate patch is sent through the RIP individually, screening starts new at each individual patch. The screening pattern produced by a RIP only repeats with the dimension of a so-called screening super-cell, so the screening of an individual patch typically will look slightly different from the screening pattern of the corresponding area in a full-format job. This can result in printing artifacts. For example, one of the separations, e.g., the black separation may be ripped full-format to provide a highly accurate mounting reference, while the other separations, e.g. cyan, magenta and yellow are cut into patches to save plate material. The screens of the full-format plate and the plate patches may then not properly match.
Another disadvantage of “electronic cutting” methods has to do with workflow. Preparing of the patches needs to be carried out in the pre-press department of the trade shop or at the printer. However, know-how of which job design should be separated into which plate patches to maximize print quality and plate making productivity typically resides only in the platemaking department. Thus inefficiencies and lack of security about final quality arise.
Finally, in many workflows, the platemaking operator has no access to editable image data. Typically, ready-made “digital film” files are sent over a high-speed network connection or over an digital medium like CD-ROM.
There thus is a need to reduce wastage of flexo plate material while using the image data produced in the pre-press department, e.g., using the screened full-format files that described the final printed result.